Gas mass flow meters are primarily known from the field of intake air mass flow measurements in internal combustion engines. Particularly good results are achieved with air mass flow meters which operate according to the principle of hot-film anemometry. This means that a heating element of the sensor is heated, wherein the generated heat of the heating element is transferred by convection to the flowing medium. The resultant temperature change of the heating element or the additional power input for maintaining the heating element temperature define a measure of the existing mass flow.
Modified mass flow sensors have in recent years also been used to measure the exhaust gas flow, as is described, for example, in DE 10 2006 058 425 A1. This device for determining the mass flow comprises two separate sensor units, wherein a first unit serves to calculate the mass flow by determining a power loss and/or a heat dissipation, and the second unit serves to determine the temperature of the exhaust gas flow. The heating element of the first sensor unit is then either controlled to an excess temperature having a constant difference to the temperature measuring element, or to a constant excess temperature. The exhaust gas mass flow can be deduced from the additional power input required for this purpose.
To prevent false measurements caused by deposits, the two sensor units comprise heating elements which can burn off contaminations at the substrates. Besides the problem of contamination in the exhaust gas line during use, there is the problem of obtaining representative measuring results in the case of pulsations and turbulences which occur to an increasing extent in the exhaust gas line. DE 10 2006 058 425 A1 therefore describes arranging two temperature measuring elements, one behind the other, which allows the recognition of the direction on the basis of the existing heat radiation from the upstream to the downstream region, which may be included in the calculation of the exhaust gas mass flow.
False measurements occur to an increasing extent during operation despite these possibilities of recognizing direction and/or pulsation and burning off deposits. A deposit at the sensor of the first sensor unit including the heating element has been identified as a cause of such false measurements, which deposit cannot be removed by being burning off, but which rather builds up on the chip due to the increased temperatures at unfavorable operating conditions, whereby inorganic compounds are produced on the surface when heating-up.
Methods for recalibrating the exhaust gas mass flow sensor during operation have therefore been created. WO 2007/075510 A1 describes a method of self-calibration of a sensor, wherein a power input of a heating element is first measured to maintain the heating element at an excess temperature relative to a temperature measuring element, a power input of the heating element is then measured while no flow exists, with the difference between the two power inputs being calculated. The heat transfer coefficient for determining the flow rate is corrected as a result. However, this method only takes into account a heat transfer by convection at the sensor account. A further, incorrect, measurement of the temperature difference due to different deposits built up at the sensor units is not taken into account.